Secondary batteries may be classified based on the structure of an electrode assembly having a cathode/separator/anode structure. For example, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which long sheet type cathodes and long sheet type anodes are wound in a state in which separators are disposed respectively between the cathodes and the anodes, a stacked type structure in which pluralities of cathodes and anodes having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the cathodes and the anodes, or a stacked/folded type structure in which pluralities of cathodes and anodes having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the cathodes and the anodes to constitute a bicell or a full cell and then a plurality of bi-cells or full-cells is folded.
In addition, secondary batteries may be classified into a cylindrical battery configured to have a structure in which an electrode assembly is mounted in a cylindrical metal container, a prismatic battery configured to have a structure in which an electrode assembly is mounted in a prismatic metal container, and a pouch-shaped battery configured to have a structure in which an electrode assembly is mounted in a pouch-shaped case made of an aluminum laminate sheet based on the shape of the battery case of each of the secondary batteries.
Particularly, in recent years, a prismatic battery having a relatively small width has been developed according to a trend of reducing the size and weight of electronic mobile devices and the prismatic battery has created applications other than the conventional cylindrical battery.
In general, the prismatic battery is manufactured by inserting some battery elements into a prismatic hollow case closed at the bottom thereof, welding a top cap assembly, injecting an electrolyte through an injection port, and sealing the injection port. The prismatic hollow case closed at the bottom thereof is generally manufactured by machining an aluminum alloy sheet using a deep drawing method as shown in FIG. 1. The deep drawing method is a typical forming method that manufactures a seamless hollow container from a flat plate. For example, a plastic deformation method that presses a flat plate 10 located on a die 30 using a punch 20 as indicated by an arrow denoted by reference numeral 21 may be used as the deep drawing method.
The deep drawing method has an advantage in that it is possible to manufacture a final hollow case from a flat plate through a series of continuous processes, thereby improving manufacturing efficiency. However, the deep drawing method has the following problems.
First, the deep drawing method includes about 10 or more complicated and sophisticated steps. For this reason, manufacturing cost of an apparatus for performing the deep drawing method is very high. In particular, it takes a long period of 2 or 3 months to manufacture a die, which greatly increases time necessary to develop a battery satisfying consumers' demand.
Second, in a case in which a metal sheet is machined using the deep drawing method, various regions of the metal sheet have different drawing degrees depending upon the shape of the die. According to circumstances, a corner rupture phenomenon may occur in a case in which drawing of the metal sheet exceeds an allowable drawing range. Consequently, shapes that can be obtained using the deep drawing method are limited.
For the above reasons, a product defect rate is high and shapes of a case are limited in a case in which the deep drawing method is used.
Consequently, there is a high necessity for a technology that is capable of fundamentally solving the above problems.